FOR a span of some dozen years early in his career, Robert M. Sapolsky, a neurologist and primatologist at Stanford University, spent three or four months a year conducting field research on baboons in Kenya. Over time, he developed an intimate appreciation of the nuances in behavior common to these highly social primates and ourselves. But it was lonely work, and Sapolsky found himself writing scads of letters back home, simply in hopes of getting some mail in return.

Thus was forged a zeal for writing in a person with no literary pretension but a great deal to write about. While in two of his previous books for a popular audience, Sapolsky focuses on his own research on the neurobiology of stress ("Why Zebras Don't Get Ulcers") and his life as a scientist ("A Primate's Memoir"), "Monkeyluv," a collection of essays published over the last 10 years in Discover, Natural History, The New Yorker and other magazines, casts a wider net. For the most part, the essays represent what Sapolsky himself describes as "hit-and-run obsessions" - topics that infect his mind for a couple of months, causing him to research endlessly and drive his poor wife to distraction with monologues on the subject until he eventually writes the obsession out of his system, leaving room for the next one.

The result of this strategy is mostly a hit. The collection is organized into three sections, each hinging on some Big Question in natural science: the relative influence of genes and environment in determining behavior; how our brains affect our bodies and vice versa; and the way society shapes the individual. Like any good modern biologist, Sapolsky, a MacArthur "genius" award winner, relies in his research on the analytical power of reductionism: the more one divides natural phenomena into their constituent parts, and those parts into subparts and so forth, the more one can learn about how nature really works. As the essays in the first section make clear, however, the notion that the genes at the base of this biological hierarchy determine the workings of everything above them is total nonsense.
In the nature versus nurture debate, Sapolsky comes down hard against the supremacy of either. As he writes in "A Gene for Nothing," for instance, actual genes make up only some 5 percent of the human genome. Among the rest of the DNA are regulatory elements that instruct the genes to turn on and off according to cues from the environment. Thus a gene "controlling" seasonal mating in a species by making proteins involved in sexual behavior might remain inactive until a warming trend in temperature activates it, whereupon "everyone starts rutting and ovulating, snorting and pawing at the ground, and generally carrying on." It isn't genes or the environment that determines behavior. It's the interaction, stupid.

If there is a flaw in the first section, it is only that this point is drilled home a bit too frequently. In the subsequent two sections, Sapolsky lets his obsessive curiosity wander amiably from topic to topic without any particular ax to grind. His aggressively vernacular style - two parts favorite campus prof, one part cruise-ship comedian - may appeal to some readers and bother others, but it is his artful articulation of the unlimited ingenuity of nature that will keep them reading. In "Bugs in the Brain," he describes a parasite that infects the brains of rats without any effect on their behavior except that they lose their instinctual aversion to the smell of cats and, instead, are drawn to it. Needless to say, such absurdly obliging prey is quickly gobbled up: bad for the rat but great for the parasite, since it can only reproduce inside a cat host. The next generation hitches a ride out on the cat's feces, which are ingested by rats to start the cycle over again.

"This is flabbergasting," Sapolsky writes. "This is like someone getting infected with a brain parasite that has no effect whatsoever on the person's thoughts, emotions, S.A.T. scores or television preferences, but, to complete its life cycle, generates an irresistible urge to go to the zoo, scale a fence, and French-kiss" the meanest-looking polar bear.

Not surprisingly, Sapolsky's quest for understanding is most compelling when the animal behavior he's reckoning with is our own. Why do rain-forest societies tend to be peaceable and polytheistic, while desert dwellers (and their cultural descendants who dominate the planet) are belligerent and believe in one God? Why does our taste for novel music, food and fashion freeze at an early age?

IN most cases, Sapolsky chases after answers to such puzzles with jovial abandon. But his very best essays have a more sober tone. "Why We Want Their Bodies Back," which explores the cross-cultural obsession with possessing some mortal remains of the deceased, begins with the aching mystery of two friends who vanished while Sapolsky was still in high school, and his own emotions when a clue to their disappearance emerged 27 years later. "Nursery Crimes," the longest essay in the book, investigates the personality disorder called Munchausen's by proxy. A distraught parent, almost always a mother, takes her sick child to the hospital, where in spite of all remedies the child's condition only seems to worsen. What the doctors and nurses do not know, of course, is that the mother is secretly poisoning the child or otherwise inducing symptoms. Sapolsky presents a chilling account of the typical M.B.P. mother - how she wins over the hospital staff with her fortitude, how she is soon taking brownies to the night crew, advising the nurses about their love lives and otherwise working her way into the social fabric of the hospital until everyone becomes an unwitting co-conspirator in her mortally pathological scheme. (The death rate of children so abused is nearly 10 percent.)

In this case, where a behavioral phenomenon so utterly violates our deepest assumptions about ourselves both as humans and as animals, Sapolsky's game pursuit of the question "why" takes us to another emotional level. Most of the essays in "Monkeyluv" are engaging. This one is a masterpiece.

Jamie Shreeve's most recent book is "The Genome War: How Craig Venter Tried to Capture the Code of Life and Save the World."